These days, with the development of high-tech products such as digital cameras, cellular phones, notebook computers, hybrid cars, and so forth, research into a secondary battery, which is rechargeable, unlike a primary battery incapable of being recharged, has extensively been conducted. As the secondary battery, a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, and a lithium secondary battery can be used. Among these batteries, the lithium secondary battery has an operating voltage of no less than 3.6 V and is employed as a power source for portable electronic devices or for a hybrid car having a high output level by connecting several lithium secondary batteries in series. The operating voltage of the lithium secondary battery is three times greater than that of the nickel-cadmium battery or the nickel-metal hydride battery, and the energy density of the lithium secondary battery per unit weight is excellent, so the lithium secondary battery has become been widely used throughout the world,
FIG. 1 is a perspective view illustrating an exemplary lithium secondary battery. The general structure of the lithium secondary battery will be described below with reference to FIG. 1.
The lithium secondary battery 10 shown in FIG. 1 is a pouch type lithium secondary battery. The lithium secondary battery 10 has an electrode assembly 20 which is also called a battery body. Metal tabs 30 project from one side of the electrode assembly 20. The electrode assembly 20 and portions of the tabs 30 are surrounded and sealed by an external finishing material 50 which is called a pouch. The external finishing material 50 is composed of a lower part 52, which defines a space for accommodating the electrode assembly 20, and an upper part 54 which complements the lower part 52. With the distal end portions of the tabs 30 exposed to the outside, the external finishing material 50 surrounds and seals the electrode assembly 20 and the remaining portions of the tabs 30. At this time, polymer films 40 are interposed between the tabs 30 and the external finishing material 50 to bond with each other and seal the tabs 30 and the external finishing material 50.
In the lithium secondary battery 10 structured as mentioned above, the sealing function of the external finishing material 50 is regarded as an important factor. If foreign substances such as moisture or the like are externally introduced into the lithium secondary battery 10, the corrosion of the electrode assembly 20, the tabs 30 and the external finishing material 50 can proceed rapidly, or the positive and negative active materials can be damaged, whereby the functionality of the lithium secondary battery 10 can be deteriorated. Generally, the tabs 30 are formed to have a thickness of 50 to 200 μm, and in this case, no moisture introduction or electrolyte leakage was found.
Meanwhile, in the lithium secondary battery 10 as mentioned above, as the output of the battery increases, the resistance and the heat generation of the tabs 30 increase. This can be confirmed from the following mathematical expression.R=r*1/s Q=i2*R  [Mathematical Expression 1]
Here, R is the resistance of the tab 30, r is the specific resistance of the tab 30, l is the length of the tab 30, s is the sectional area of the tab 30, Q is the heat generation of the tab 30, and i is a current.
Accordingly, the sectional area of the tab 30 can be increased in order to increase the output of the battery and decrease the heat generation of the tab 30. However, this method has a problem in that the thickness of the tab 30 is increased and the sealing function between the external finishing material 50 and the tabs 30 is consequently degraded.
FIG. 2 is a perspective view illustrating a state in which a tab 30′ and the polymer films 40 are coupled with each other in accordance with the conventional art, and FIG. 3 is a cross-sectional view taken along the line L1-L1 of FIG. 2. As can be readily seen from FIGS. 2 and 3, in the case that the thickness d of the tab 30′ increases up to no less than a predetermined value, for example, 200 μm, the polymer films 40 which surround the tab 30′ cannot be brought into close contact with the side surfaces of the tab 30′, as a result of which a gap A is created between the polymer films 40 and the side surface of the tab 30′. That is to say, as the external finishing material 50 and the tab 30′ are not completely sealed, external moisture may be introduced into the lithium secondary battery through the gap A, or electrolyte may leak outside through the gap A.